restore default audio output position first in the enumerateDevices() returned list

[karlt]

w3c/mediacapture-main#1057 removed the ability to identify the system default audio output device, and to detect when the system default audio output device changes, from the ordering of audio output device info objects, added for #113.

This removal was not explained in the pull request, commit messages, or #151, so I assume it was unintentional.

Some sites use knowledge of the default audio output device to tell the user which audio output device is used for producing sound, which is particularly helpful when that may not otherwise be clear.

[karlt]

Proposal:
When the physical device that is currently the system default audio output is exposed, order the "list of device info objects" such that this physical device is the first "audio-output" object, before the new deviceId:"default" object for the virtual device.

This would provide an ordering that is distinguishable from that with a placeholder virtual device info object for #133, which would be the first in the list when the currently default physical device is not exposed.
Although w3c/mediacapture-main#1057 added a default virtual device only when the currently default physical device is exposed, #133 proposes adding a similar virtual device when the currently default physical device is not exposed.
If the virtual device were in the first position in both cases, then the position of the second "audio-output" device info object would not necessarily mean that it is the currently default physical output device.

[youennf]

This point was actually discussed at last editor's meeting.
The spec does not mandate to make the default output device second in the list (after the default output device) but UA can do it. The spec could mandate it by prepending instead of appending the default output device entry.

Safari does it for instance but I haven't tested what other UAs are doing. It would be worth checking what Chrome is doing.
We could try enforcing this ordering.

groupId can also help to make sure to relate the two entries. The spec could explicitly state that the two entries should have the same groupId.

[karlt]

For a sample of two desktop systems with Google Chrome:

• Neither microphones nor speakers were ordered with the default physical device first before other physical devices of the same kind.
• The virtual id:"default" microphone was last on one system and first on another.
• The virtual id:"default" microphone and speaker had groupIds matching default physical devices on one system but not on the other.
• The label on the id:"default" virtual devices included text for the physical device on one system ("Default - MacBook Pro Speakers (Built-in)") but not the other.

groupId would not be a dependable way to identify the default physical device because two physical devices of the same kind can have the same groupId.